The present invention relates to a method and apparatus for the control of an ignition system spark timing for a spark ignition internal combustion engine, and more particularly to a method and apparatus for the control of an ignition system spark timing during cranking of a spark ignition internal combustion engine.
Description is now provided concerning the conventional art and the problems associated therewith. The spark timing control device of an internal combustion engine adjusts the timing of spark to a certain degree before the top dead center (for example, 12.degree. C. BTDC/600 rpm) so as to cause the ignition to take place at the optimum instance to provide the maximum output because there is an ignition delay from the passage of a spark to the actual ignition of the fuel within the cylinder of the engine. The ignition delay in terms of time is substantially unchanged even if the engine revolution speed increases, but if it is considered in terms of crank angle, the ignition delay increases in accordance with an increase in the engine revolution speed because the piston speed increases in accordance with the increase in engine revolution speed. Thus, a centrifugal spark advance unit of the mechanical type, for example, is operatively connected to a distributor, thereby to advance the spark timing in accordance with an increase in the revolution speed because if the spark timing remains at the same level as that for the low revolution speed, an adequate output characteristic can not be provided when the revolution speed increases.
FIGS. 1 and 2 are graphs showing spark advance characteristics provided by the conventional devices wherein the axis of abscissa designates engine revolution speed and the axis of ordinate designates spark advance value. The reference letter N.sub.1 designates a revolution speed beyond or above which the spark advance value starts to increase, and the letter N.sub.2 designates a revolution speed beyond or above which the spark advance value becomes maximum. In the case of FIG. 1, the characteristic features in that the spark advance value linearly increases as the revolution speed increases when the revolution speed is between N.sub.1 and N.sub.2. In the case of FIG. 2, the characteristic features in that the gradient of a slope between N.sub.1 and N.sub.2 changes at N.sub.3. In both of the cases, spark advance values within a low revolution speed range below N.sub.1 and within a high revolution speed range above N.sub.2 are constant, thereby to maintain the spark advance constant irrespective of the variation in the revolution speed. The letter N.sub.4 designates the idle revolution speed. As will be understood from an observation of the characteristics shown in FIGS. 1 and 2, according to the conventional devices, the spark advance value remains constant when the engine is under cranking, so that there is a problem that if the cranking speed is low, the ignition resulting from the passage of a spark causes a torque tending not to rotate the engine crankshaft in the forward direction, but to rotate in the reverse direction, thereby to deteriorate the engine start-up performance when the atmospheric temperature is low and the battery voltage is low.